Last year I cleaned up my firebox and wirebrushed everything down to clean metal. instead of painting I just used stoveblack and polished it up with several coatings. looked great for about three weeks but it turned white and flaked off. One thing I worry about is these fireboxes seem a bit thin, are there concerns about it rusting through ? I thought about refractory cement or furnace cement and coating the back wall with a thin coating but think this may screw with the transfer of heat. The next thing is using a high temp primer and coating and just replace each year. Anyone have an idea what the max temp range is in the firebox ?

If you could make the firebox perfect the only place that would take heat from it would be the heat exchanger. The sides and top are insulated to keep the heat in. The back of the firebox is protected by a fireboard that radiates heat through the door glass and reduces heat from reaching the back of the firebox. To protect the steel during the non-heating season clean the metal and coat it with a spray on rust preventer.